Method of manufacturing heat absorbing glass

ABSTRACT

An improved method of making a ferrous ion-containing, heat absorbing glass article is disclosed. The improvement resides in incorporating iron and tin in the glass, and establishing and maintaining during the glass melting, refining and forming period at least 80 percent of the iron in the glass in the ferrous state. Further, sufficient tin is maintained in the stannous state to act as an internal reducing agent to prevent oxidation of the ferrous ion to the ferric species in a subsequent glass reheating step.

United States Patent Janakirama-Rao Dec. 18, 1973 METHOD OFMANUFACTURING HEAT 3,278,319 10/1966 Cohen 65/32 x ABSORBING GLASS3,307,929 3/1967 Trap 65/32 2,469,490 3/1949 Armistead... 65/134 X [75]Inventor: Bhogaraiu V- Jana rama-Ra 3,345,190 10/1967 Albinak et al. 6532 x Aspinwall, Pa. 3,231,456 1/1966 McMillan et a]. 65/32 X O 4 1969 Lk l [73] Ass1gnee: PPG Industries, Inc., Plttsburgh, Pa. 811970 ir g i iI g 22 Filed: No 3 1971 3,649,311 3/1972 Arauju 106/54 [21] APPL N05203,404 Primary Examiner-Frank W. Miga Related U.S. Application Dataflimsy-Russell Eberly [63] Continuation-impart of Ser. No. 5,796, Jan.26,

1970, Pat NO. 3,652,303. [57] ABSTRACT An improved method of making aferrous ion- [52] U.S. C1. 65/32, 65/134, 65/30, ing, a orbing gl rticleis disclosed. 106/54 The improvement resides in incorporating iron andtin [51] lint. C1. C03b C03b 5/16, C03b 3/04 in the g and ing n m i n nguring [58] Field 61 Search 65/32, 134; 106/52, the glass melting,refining and forming Period at least 106/54 80 percent of the iron inthe glass in the ferrous state. Further, sufficient tin is maintained inthe stannous [56] References Cited state to act as an internal reducingagent to prevent UNITED STATES PATENTS oxidation of the ferrous ion tothe ferric species in 21 2,505,001 4/1950 Nordberg 65/32 x Subsequentglass reheatmg Step 2,612,727 10/1952 Nordberg 65/32 15 Claims, 1Drawing Figure RELATIVE rRANm/rm/cz 41' 0.25" Til/cults:

' I I 1 1 I 1 1 1 1 1 1 CLEAFZPLATE 8 8 J 3 7o l 1 g 60 2 I i g I0251511 TINTED PLATE 1"J fl 40 I 1 g I BLUE GLASS 1 30 I 1 1 20 f 1 1 101 1 1 l 1 1 300 400 500 600 700 800 1000 I200 H001600 15 0 Z000 Z200WAVELENOTH 1N MILLIM l CIZONS METHOD OF MANUFACTURING HEAT ABSORBINGGLASS CROSS REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of my copending application Ser. No. 5,796, filedJan. 26, 1970, now U. S. Pat. No. 3,652,303, issued Mar. 28, 1972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isin the field of heat absorbing glasses. More particularly, thisinvention is in the field of high visible light transmitting and highinfrared radiation absorbing glasses containing ferrous ions, whichglasses are suitable for automobile and architectural applications.

2. Description of the Prior Art In the manufacture of heat absorbingautomotive and architectural glasses, iron, usually in the form offerric oxide (Fe O is added to the glass composition. The iron in glassunder ordinary conditions of melting in air is in two states, theferrous state (Fe) and ferric state (Fe with an equilibrium establishedaccording to the equation:

In glass melted at 2,6002,900F. in air, this equilibrium is found to bein the range of 40 to 60 weight percent Fe with respect to the totaliron. It is well established in the art, see for example, U.S. Pat. No.3,326,703, that it is the presence of the ferrous species which givesrise to heat absorption. Ferrous ion absorbs infrared radiation becauseit has an absorption band in the near infrared region of the spectrum,around 1,100 millimicrons wavelength. The magnitude of the infraredabsorption band of the ferrous ion at its maximum around 1,100millimicrons is a function of the concentration of the ferrous ion.Therefore, the greater the ferrous ion concentration, the greater theamount of heat the glass will absorb. However, ferrous ion gives theglass a blue color in the visible regions of the spectrum and cuts downon visible light transmission and limits its permissible concentrationin glass. For automobile applications, an infrared radiation absorbingglass has to transmit greater than 70 percent visible light at a A-inchthickness. Thus, a dilemma is created in that it is desirable to havehigh ferrous ion concentrations to absorb a high percentage of theincoming infrared radiation, yet the ferrous and ferric ionconcentration must not be so great that the visible light transmissionis too greatly reduced.

This dilemma is solved by the improved method of this invention. Theinvention provides for the incorporation of a relatively small amount ofiron in the glass, thus insuring a high visible light transmission.Besides, at least 80 percent of the iron is converted to the ferrousstate, thereby providing a high infrared absorbing glass. The iron ismaintained in the ferrous state by adding to the glass composition a tincompound. Sufficient tin is maintained in its stannous state in theresultant glass to serve as an internal reducing agent in the glass andprevent any tendency towards conversion of the ferrous ion to the ferricspecies arising from oxidizing influences, such as oxygen diffusion fromair into the glass when the glass is reheated during a subsequentfabrication step, as for example, press bending or thermal tempering.

SUMMARY OF THE INVENTION In accordance with this invention, there isprovided an improved method of making a ferrous ioncon taining, heatabsorbing glass article, which includes the steps of mixing, melting,refining, forming and fabricating glass, and which method includes,during the fabrication step, a subsequent reheating of the formed glass,the improvement comprising:

a. incorporating iron and tin in the glass, and

b. establishing and maintaining in the glass prior to reheating i. atleast percent by weight of the iron in the ferrous state, and I ii.sufficient tin in the stannous state to prevent oxidation of the ferrousion in the subsequent reheating of the formed glass.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows the spectraltransmittance curves between wavelengths of 300 and 2,000 millimicronsof three %-inch thick samples of glass. The glass samples are clearplate glass, green-tinted plate glass and bluetinted plate glass, whichhas been prepared by the improved method of this invention.

DETAILED DESCRIPTION As has been mentioned above, the invention isdirected to an improvement in a method of making a ferrousion-containing heat absorbing glass article.

In the making of glass articles, the various ingredients 'which go intothe glass are first batch mixed and handled in a dry or wet state. Theglass batch in powder, pellet or wet mix form is charged to a furnacewhere the ingredients are melted and reacted with one another. Thetemperature of the glass in the melting furnace is about 2,600-2,900F.During the melting and reacting, gases are being continuously evolved,and sometimes they become entrapped within the glass melt. To removethese pockets of gas, the glass melt is passed to a lower temperaturesection of the tank, the so-called refining section, where the glass isrefined at a temperature of about 2,400F. The refining operation makesthe glass clear, bubble-free and homogeneous. Following the melting andfining, the glass is flowed from the refining portion of the tank andformed into flat glass. The forming operation consists of taking thehot, viscous liquid glass, which is at a temperature now of aboutl,5002,000F. and shaping it by pressing, drawing, rolling or floatingthe glass into flat sheet form.

Immediately after being formed, the glass is conveyed to a long oven,called a lehr, in which the flat glass is annealed. After annealing, theglass is cooled to room temperature. The glass can then be reheated in asubsequent fabrication step to make a resultant glass article. Forexample, when the glass is to be press bent into a curved motor vehicleclosure, the glass is usually reheated to a temperature of aboutl,100-1,200F. prior to bending. As another example, if the glass is tobe thermally tempered, it must first be heated to a temperaturesomewhere near its strain point; for soda-lime silica glass, this isabout 1,400F.

It is in these subsequent reheating steps, after forming, that oxidativeinfluences are greatest. The oxidative influences are believed to be dueto the diffusion of oxygen from the air into the glass during thereheating. 1n manufacturing a heat absorbing glass containing ferrousion, these oxidizing influences may convert a significant portion of theferrous ion to the ferric species, which reduces the heat absorbingcapacity of the glass. The improved process of the invention overcomesthis problem and provides for establishing and maintaining throughoutthe glass melting, refining and forming period at least 80 percent ofthe iron in the ferrous state and sufficient tin in the stannous stateto prevent oxidation of the ferrous ion to the ferric species in asubsequent reheating of the formed glass article.

The invention can be accomplished by adding iron and tin to the glassbatch as metallic iron and metallic tin. The metallic iron and tintogether counteract the oxidizing influences of the refining agents (forexample, sodium chloride, sodium nitrate and arsenious oxide) used inthe glass composition and, in the process,

are converted themselves to ferrous and stannous ions, respectively. Thepresence of stannous ion in the glass serves as an internal reducingagent and prevents subsequent oxidation of the ferrous ion to the ferricspecies in a subsequent fabrication step where the glass has to bereheated. Any tendency towards conversion of the ferrous ion to theferric species arising from oxidizing influences is efficientlyprevented by the presence of stannous ion which pushes the equilibriumbetween the ferric and the ferrous ions towards the ferrous state, as isshown in the equation below:

This is so because the stannous ion is a more powerful reducing agentthan the ferrous ion.

Besides the iron and tin being added to the glass batch in their fullyreduced, metallic states (Fe and Sn respectively), they may be added tothe glass batch in their higher oxidative states. The iron may be addedin its ferrous state (Fe) and even in the ferric (Fe state. Exampleswould include the addition of iron in the form of ferrous oxide orferrous oxalate and as ferric oxide. The tin may be added in thestannous (Sn) and stannic (Sn states as stannous chloride and stannicchloride, respectively. If the iron and tin are added in their higheroxidation states, certain precautions must be taken to insure that atleast 80 percent of the iron is converted and is maintained throughoutthe manufacturing period in the ferrous state, and sufficient tin ispresent in the glass composition in the stannous state to preventoxidation of the ferrous ion in a subsequent reheating step. Suchprecautions would include, for example, using a reducing atmosphere inthe melting and fining steps of a glass manufacturing process. The useofa reducing atmosphere will convert the iron present in the ferricstate to the ferrous state. The reducing atmosphere can be attained, forexample, by bubbling hydrogen or natural gas through the glass melt.Alternately, a constituent could be added to the glass compositionwhich, during the melting and refining operations, would decompose,releasing hydrogen. Such a constituent would be, for example, ammoniumchloride or oxalic acid, which decompose at the glass meltingtemperature and release hydrogen gas.

With regards to the amount of iron and tin which are to be incorporatedin the glass composition in practicing the improved method of thisinvention, the amounts incorporated should, of course, be sufficient toprovide a ferrous ion content measured as FeO in the resultant glasscomposition of between 0.05 and 2.60 percent by weight based on totalweight of the glass composition, and a stannous ion content measured asSnO of between 0.10 and 5.70 percent by weight based on total weight ofthe glass composition. There should be sufficient tin maintained in thestannous state to act as an internal reducing agent to prevent oxidationof the ferrous ion to the ferric species when the glass is reheated in asubsequent reheating step to fabricate a glass article. For thispurpose, the mole ratio of stannous ion to ferrous ion before thereheating step should be at least 1:1. The proportion of FeO can bemeasured by spectrophotometric analysis. If the amount of FeO in theglass composition is less than 0.05 percent by weight, the heatabsorption of the glass is lower than desired. 1f the FeO content in theglass composition is present in excess of 2.60 percent by weight,visible light transmission is unsatisfactorily low. If the amount of SnOin the glass composition is less than 0.1 percent by weight, then therewill be insufficient stannous ion present in the glass to preventsubsequent oxidation of the ferrous ion in a reheating step afterforming of the glass.

in general, it has been found that the above results with regards to thepercentages of FeO and SnO can be achieved if from about 0.05 to 2percent by weight iron measured as Fe and 0.1 to 5 percent by weight oftin measured as Sn are added to the glass batch. Percentages by weightare based on the total weight of the glass batch. If greater amounts ofiron are added than those specified, the glass will have an undesirablylow visible light transmittance. 1f lower amounts of iron are used thanthat recommended, the glass will not have the high infrared absorptionproperties. lftoo much tin is added, that is, greater than 5 percent byweight, then the working temperature range of the glass will beundesirably narrow, such that the improved method of the inventioncannot be used with the currently employed commercial processes formaking flat glass. If less than the required amount of tin is used, thatis, less than 0.1 percent by weight, then there will be insufficientstannous ion present in the glass to prevent subsequent oxidation of theferrous ion in a subsequent reheating step.

Examples of the various commercial glass manufacturing processes whichcan employ the improvement of this invention are the float process formaking float glass, as described in US. Pat. No. 3,083,551; thePittsburgh Process, also known as the PENNVERNON Process (registeredtrademark of PPG Industries, lnc.) to make sheet glass, such processbeing described in US. Pats. Nos. 1,833,379, 1,833,380, 1,833,381 and1,833,382; and the Continuous Plate Process for making plate glass, asdescribed in US. Pats. Nos. 2,478,090 and 2,505,103. Suitable glasscompositions which can be used in the practice of this invention are asfollows:

Weight Based on Total Component Weights as Determined by Wet ChemicalAnalysis Components Permissive Range Preferred Range SiO; -80 68-74 Na,O10-20 11-14 K 0 0-10 0-1 Na O K 0 10-21 11-15 CaO 5-16 6-13 MgO 0-101.5-4 CuO MgO 5-20 10-15 A1 0, ()-5 1-3.5 1 0,0 0.1-5.11 0.20-55 SnO,0-117 0-1 .2

SnO 0.1-5.00 0.1-5.00 Cl 0.1-2 0.l-l.5 Total iron in the glasscomposition measured by wet chemical analysis as Fe O Weight Based onTotal Component Weight as Determined by Absorption SpectrophotometricAnalysis Components Permissive Range Preferred Range Pep, 0-l.l4 Ol.l0FeO 0.06-2.57 0.06-2.57

The glass former is SiO Sodium oxide is present as a flux to reduce themelting temperature of the glass. Potassium oxide may be employed inplace of a portion of the Na O, but the use of Na O is preferred becauseit is less expensive. The total amount of alkali metal oxide in theglass should range from to 21 percent by weight. ,CaO and MgO are alsoemployed as fluxes. They are used to supplement the Na O because theyimprove the chemical durability of the glass. The amount of alkalineearth metal oxides in the glass can range from 5 to percent by weight.Alumina may be present in the glass in varying amounts, depending mainlyon the manner in which the glass is formed. Alumina is employed toregulate the viscosity of the glass, improve its durability and preventdevitrifieation of the glass. Relatively small amounts of alumina, forexample, less than 1.0 percent by weight, are employed when plate glassis cast from a pot or made in a tank and formed by rolling ithorizontally between sizing rolls. However, when the glass is formed bydrawing it vertically from the kiln of a tank over a draw bar andupwardly between rollers (sheet glass), up to 5 percent by weightalumina can be employed. Usually, however, from 1.0 to 3.5 percent byweight alumina is employed when forming glass by the sheet or verticaldrawing procedure. Trace amounts of Ti0 are frequently present, e.g., inthe amounts of up to 0.05 percent by weight, as an impurity.

The glass articles prepared by the improved method of this invention canbe produced from conventional glass making materials properly compoundedand thoroughly mixed so as to yield, when reacted, glasses of thedesired composition. Suitable batch materials include sand, soda ash,potassium carbonate, limestone, dolomite, aluminum hydrate, aplite,feldspar, rouge, ferrous oxide, stannous oxide, metallic iron andmetallic tin. Refining agents, such as salt cake, sodium nitrate, sodiumchloride, antimony oxide, arsenious oxide and combinations thereof, maybe present in the batch provided certain precautions are taken. Therefining agents are strong oxidizers and when they are used, care shouldbe taken that a reducing atmosphere is maintained in the melting andrefining stages of the manufacturing process. Also, if refining agentsare used, the iron and tin should be added in their reduced forms asmetallic iron and metallic tin.

The glass compositions manufactured in the improved process of theinvention have viscosity versus temperature characteristics suitable fordrawing into flat glass by modern plate, sheet and float glassproduction methods.

Also, the glass articles manufactured according to the improved processof this invention have high visible light transmittance and low infraredradiation transmittance. Accordingly, the visible light transmittance(i.e., between wavelengths of 350 to 500 millimicrons) is greater than70 percent at A-inch thickness, and the infrared radiation (i.e.,between wavelengths of 950 and 2,000 millimicrons) transmittance is lessthan 15 percent at A-inch thickness. In addition, the glass of thisinvention has the total solar energy transmittance of less than 50percent at 54-inch thickness, which complies with Federal SpecificationNo. D-G-45 l -A for automotive heat absorbing Windshields.

EXAMPLES The following examples depict the improved method of thisinvention. in the various examples, iron and tin are added to the glasscompositions in differently respective oxidation states. In eachexample, however, the method of manufacture converts at least percent ofthe iron to the ferrous state to give a pale, bluecolored glass withhigh visible light transmittance and high infrared radiation absorption.Also, sufficient tin is maintained in the stannous stateto preventoxidation of the ferrous ion to the ferric species.

Example I This example shows the addition ofiron and tin to the glasscomposition in their respective metallic states.

Parts by Weight Batch Ingredients (in grams) Sand 7H) Sodium carbonate205 Calcium carbonate I52 Magnesium carbonate 80 Al(OH), 3 Tin metal,200 mesh 20 Iron metal, 200 mesh 3.5 Sodium sulfate 35 Sodium chloridel0 Oxalic acid 5 A laboratory pot-glass batch is prepared by thoroughlymixing the above batch ingredients to give a 1,223.5 gram batch. Thisbatch yields approximately 1,000 grams of glass after melting and finingwhich is conducted as follows.

An empty refractory clay pot is heated in a gas-fired furnace at afurnace temperature of about 2,900F. A portion (approximately one-third)of the mixed batch is ladled into the preheated pot at the furnacetemperature of about 2,700F. The remaining portion of the mixed batch isladled into the pot over a period of 2 hours, and the temperature israised gradually to about 2,900F. during this time. The temperature ismaintained at 2,900F. for the next 4 hours to refine the glass. At theend of this time, the chemical reactions are completed; the glass isfree of bubbles and is substantially homogeneous.

After the glass is refined, the temperature of the furnace is reduced to2,700F. over a period of 1 hour. The furnace is then held at thistemperature for 1 hour. The pot is removed from the furnace, and theglass is poured on a metal table and rolled in the form of a plate. Theplate is placed in a kiln and cooled from 1,200F. to room temperature.The glass is subsequently cut into pieces suitable for grinding,polishing and testing. The calculated composition of the abovegivenbatches is as follows:

We! Chemical Analysis Percent by Weight Based Component on TotalComponent Weight SiO, 7L0 N3 0 CaO 8 5 MgO 3.8 Ano 0.2 SnO, 2.5 Fc of0.5

CI O.l Total iron and tin in the glass composition measured by wetchemical analysis as Fc O and SnO respectively.

Absorption Spectrometric Analysis Component Percent by Weight Fe o 0.01FeO 0.21

A 41-inch thick polished plate sample of glass made by the above potmethod, when subjected to optical and spectral transmittance, yieldedthe following values:

Property Percent Luminous Transmittancc 74 Total Solar InfraredTransmittance l Total Solar Energy Transmittance 39 Examples II V Thefollowing examples depict, on a commercial scale, the improved method ofthe invention. In the examples, iron is added to the glass batch in theform of ferric oxide, and tin is added to the glass batch in the form ofstannous chloride. Also, ammonium chloride and oxalic acid are includedin the glass batch to give a reducing condition internally, since theydecompose in the glass melt to release hydrogen. In the method depictedby these examples, greater than 80 percent of the iron is converted tothe ferrous state, and sufficient tin is maintained in the stannousstate to prevent further oxidation of the ferrous ion.

A 3,600-pound glass batch was prepared by thoroughly mixing the variousingredients listed below:

Batch Ingredients Parts by Weight Sand 730 Soda ash 235 Limestone 57.4

Dolomite 9| Ferric oxide 2.5

Oxalic acid 8 Ammonium chloride l2 In the above batch, metallic ironpowder can be substituted for ferric oxide and/or metallic tin could besubstituted for SnCl -2H O. As an added means of attaining reducingconditions, hydrogen gas or'natural gas could be bubbled through themolten glass.

In melting the above batch, a refractory pot is preheated to 2,600F. ina gas-fired furnace. The above batch is added over a -hour period infive intervals. The temperature is then raised to 2,700F., and the glassis stirred at this temperature for 10 hours. The stirrer is removed, andthe glass is refined 5 hours at 2,600F. to remove the entrapped bubblesof gas. The furnace temperature is then dropped to 2,300F., and theglass is held at this temperature for 6 hours before casting. The glassmelt is then cooled to the casting temperature of 2,100F. Finally, theglass which is now free of bubbles is ready to be cast. The glass iscast onto a sheet rolling machine and rolled in the form of a plate. Theplate is conveyed into a lehr and cooled from l,200F. to 200F.Thereafter the glass is cooled more rapidly to room temperature andsubsequently cut into pieces suitable for grinding, polishing andtesting. The above commercial pot batch yields approximately 3,000pounds of glass after melting and refining, which amounts to 700 squarefeet of plate glass at 5/16-inch thickness.

The composition of the above-given batch is as follows:

Wet Chemical Analysis Calculated Percent Com- Percent by Weight Based byWeight Based on ponent on Total Component Weight Total Component WeightsiO 70.l8 Na O I246 12.5 CaO 8.26 8.5 MgO 3.16 3.3 AI,O I.40 1.25 SnO,*3.36 3.0 Fe Of 0.26 0.25 CI 0.92 L20 Total iron and tin in the glasscomposition measured by wet chemical analysis as Fc O and SnOrespectively.

Absorption Spectrophotometric Analysis Component Percent by Weight 1 30.027 FeO 0.206

A A-inch thick polished plate sample of glass made by the pot method,when subjected to optical and spec tral transmittance measurements,yields the following values:

Property Percent Visible Light Transmittance as Measured by LuminousTransmittance Total Solar Infrared Transmittance Total Solar EnergyTransmittance Pounds Percent Percent Fc O Percent Total Solar TotalSolar Glass added to Luminous Infrared Energy No. Glass Trans-Transmitrance Transmittance Batch mittance II 2.5 74.l 10.9 39.8 I 2.075.5 13.6 42.l IV 3.0 72.9 8.5 37.9 V 3.5 70.4 6 I 352 Glass numbers IIto V were melted according to the preferred embodiment of the invention.

Glass number II is the blue glass shown in the accompanying drawing. Thetable shows how luminous transmittance decreases and heat absorptionincreases with increasing iron content.

The luminous transmittance is the summation of the percentage of theincident visible radiant energy (weighed by the energy distribution ofthe source and the eyes sensitivity) that will pass through the glass asdescribed. In the invention, the source is illuminant C, a standardsource adopted by the International Commission on Illumination.

The total solar energy transmittance value as reported herein arecalculated from the spectral transmittance of the glass and the spectraldistribution of the solar radiant energy. The latter values are those ofParry Moon (Journal oflhe Franklin Institute, volume 230, NovemberI940).

The spectral transmittance measurements of the glasses of the presentinvention were made by standard spectrophotometric methods. A BeckmanQuartz Spectrophotometer, Model DK-2A, is used for wavelengths between300 and 2,300 millimicrons.

It should be appreciated, of course, that various size pots or cruciblescan be employed in the practice of this invention, and the meltingtemperatures and times will vary according to the amount being formed.Also, the glass compositions used in practicing the method of thisinvention have viscosity versus temperature characteristics which aresuitable for drawing into flat glass by modern plate, sheet and floatglass mass production methods.

In the above examples, the total tin content of the glass samples wasmeasured as SnO although it should be realized that appreciablequantities of SnO are present in the glass. To determine the SnOcontent, a weighed glass sample is dissolved in a mixture ofconcentrated hydrofluoric and hydrochloric acids and a saturated boricacid solution. The solution is treated with potassium iodate whichoxidizes the stannous ion to stannic. Based on the amount of potassiumiodate needed, the stannous oxide content of the glass composition canbe determined.

1 claim:

1. A method of making a high infrared radiation absorbing, high visiblelight transmitting, soda-lime-silica flat glass article useful for motorvehicle Windshields and architectural applications, comprising:

a. preparing a glass batch composition by adding to a batch ofsoda-lime-silica glass making ingredients,

i. 0.05 to 2.0 percent by weight iron measured as Fe"; the iron beingadded in the form of metallic iron or compounds of ferrous or ferricions to provide a ferrous-ferric equilibrium upon the melting of saidglass batch composition,

ii. 0.1 to percent by weight tin measured as Sn; at least part of thetin being added in the form of metallic tin to provide stannous ionsupon the melting of said glass batch composition;

b. melting the glass batch composition to form a glass melt so that aninternal reduction occurs to shift the normally occurring ferrous-ferricion equilibrium to the ferrous state;

c. forming a flat glass article from said glass melt in which at least80 percent of the total iron in the glass is the ferrous state, and themole ratio of stannous ions to ferrous ions is at least 121, thestannous ion serving as an internal reducing agent in the glass; and

d. cooling said flat glass article.

2. The method of claim 1 in which a reducing gas is bubbled through theglass melt.

3. The method of claim 2 in which the reducing gas is hydrogen ornatural gas.

4. The method of claim 1 in which an ingredient is added to the glassbatch which upon melting releases a reducing gas in the glass melt.

5. The method of claim 4 in which the ingredient is ammonium chloride oroxalic acid.

6. The method of making a high infrared radiation absorbing, highvisible light transmitting, soda-limesilica flat glass article usefulfor motor vehicle windshields and architectural applications,comprising:

a. preparing a glass batch composition by adding to a batch ofsoda-lime-silica glass making ingredients,

i. 0.05 to 2.0 percent by weight iron measured as Fe"; the iron beingadded in the form of metallic iron or compounds of ferrous or ferricions to provide a ferrous-ferric equilibrium upon the melting of saidglass batch composition,

ii. 0.1 to 5 percent by weight tin measured as Sn"; at least part of thetin being added in the form of compounds of stannous ions;

b. melting the glass batch composition to form a glass melt so that aninternal reduction occurs to shift the normally occurring ferrous-ferricion equilibrium to the ferrous state;

c. forming a flat glass article from said glass melt in which at leastpercent of the total iron in the glass is in the ferrous state, and themole ratio of stannous ions to ferrous ions is at least 1:], thestannous ion serving as an internal reducing agent in the glass; and

d. cooling said flat glass article.

7. The method of claim 6 in which a reducing gas is bubbled through theglass melt.

8. The method of claim 7 in which the reducing gas is hydrogen ornatural gas.

9. The method of claim 6 in which an ingredient is added to the glassbatch which upon melting releases a reducing gas in the glass melt.

10. The method of claim 9 in which the ingredient is ammonium chlorideor oxalic acid.

11. The method of making a high infrared radiation absorbing, highvisible light transmitting, soda-limesilica'flat glass article usefulfor motor vehicle windshields and architectural applications,comprising:

a. preparing a glass batch composition by adding to a batch ofsoda-lime-silica glass making ingredients,

i. 0.05 to 2.0 percent by weight iron measured as Fe; the iron beingadded in the form of metallic iron or compounds of ferrous or ferricions to provide a ferrous-ferric equilibrium upon the melting of saidglass batch composition,

ii. 0.1 to 5 percent by weight tin measured as Sn; at least part of thetin being added in the form of compounds of stannic ions;

b. melting the glass batch composition to form a glass melt;

c. introducing a reducing agent into said glass melt to convert thestannic ions in said melt to stannous ions and to shift the normallyoccurring ferrousferric ion equilibrium to the ferrous state;

d. forming a flat glass article from said glass melt in which at least80 percent of the total iron in the glass is in the ferrous state, andthe mole ratio of stannous ion to ferrous ion is at least lzl, thestannous ions serving as an internal reducing agent in the glass; and

e. cooling said flat glass article.

12. The method of claim 11 in which the reducing agent is a gas bubbledthrough the glass melt.

13. The method of claim 12 in which the reducing agent is hydrogen ornatural gas.

14. The method of claim 11 in which an ingredient is added to the glasswhich upon melting releases a reducing gas in the glass melt.

15. The method of claim 14 in which the ingredient is ammonium chlorideor oxalic acid.

2. The method of claim 1 in which a reducing gas is bubbled through theglass melt.
 3. The method of claim 2 in which the reducing gas ishydrogen or natural gas.
 4. The method of claim 1 in which an ingredientis added to the glass batch which upon melting releases a reducing gasin the glass melt.
 5. The method of claim 4 in which the ingredient isammonium chloride or oxalic acid.
 6. The method of making a highinfrared radiation absorbing, high visible light transmitting,sodA-lime-silica flat glass article useful for motor vehicle windshieldsand architectural applications, comprising: a. preparing a glass batchcomposition by adding to a batch of soda-lime-silica glass makingingredients, i. 0.05 to 2.0 percent by weight iron measured as Fe0; theiron being added in the form of metallic iron or compounds of ferrous orferric ions to provide a ferrous-ferric equilibrium upon the melting ofsaid glass batch composition, ii. 0.1 to 5 percent by weight tinmeasured as Sno; at least part of the tin being added in the form ofcompounds of stannous ions; b. melting the glass batch composition toform a glass melt so that an internal reduction occurs to shift thenormally occurring ferrous-ferric ion equilibrium to the ferrous state;c. forming a flat glass article from said glass melt in which at least80 percent of the total iron in the glass is in the ferrous state, andthe mole ratio of stannous ions to ferrous ions is at least 1:1, thestannous ion serving as an internal reducing agent in the glass; and d.cooling said flat glass article.
 7. The method of claim 6 in which areducing gas is bubbled through the glass melt.
 8. The method of claim 7in which the reducing gas is hydrogen or natural gas.
 9. The method ofclaim 6 in which an ingredient is added to the glass batch which uponmelting releases a reducing gas in the glass melt.
 10. The method ofclaim 9 in which the ingredient is ammonium chloride or oxalic acid. 11.The method of making a high infrared radiation absorbing, high visiblelight transmitting, soda-lime-silica flat glass article useful for motorvehicle windshields and architectural applications, comprising: a.preparing a glass batch composition by adding to a batch ofsoda-lime-silica glass making ingredients, i. 0.05 to 2.0 percent byweight iron measured as Fe0; the iron being added in the form ofmetallic iron or compounds of ferrous or ferric ions to provide aferrous-ferric equilibrium upon the melting of said glass batchcomposition, ii. 0.1 to 5 percent by weight tin measured as Sn0; atleast part of the tin being added in the form of compounds of stannicions; b. melting the glass batch composition to form a glass melt; c.introducing a reducing agent into said glass melt to convert the stannicions in said melt to stannous ions and to shift the normally occurringferrous-ferric ion equilibrium to the ferrous state; d. forming a flatglass article from said glass melt in which at least 80 percent of thetotal iron in the glass is in the ferrous state, and the mole ratio ofstannous ion to ferrous ion is at least 1:1, the stannous ions servingas an internal reducing agent in the glass; and e. cooling said flatglass article.
 12. The method of claim 11 in which the reducing agent isa gas bubbled through the glass melt.
 13. The method of claim 12 inwhich the reducing agent is hydrogen or natural gas.
 14. The method ofclaim 11 in which an ingredient is added to the glass which upon meltingreleases a reducing gas in the glass melt.
 15. The method of claim 14 inwhich the ingredient is ammonium chloride or oxalic acid.